Need to know how to measure windows? Use this window measurement guide.

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How to measure windows

Learn How to Measure Windows

Replacing windows in your home doesn’t have to be a difficult process as long as you know how to measure properly. Taking an accurate measurement helps ensure that any replacement you purchase, or even a brand new window is the proper size. Learn and follow the right steps to get a good measurement so you get great results when installing windows on your home.

How to Properly Measure the Rough Opening

It’s vital that new windows fit into the openings that you have available, so you must measure carefully. In order to determine the right size windows for your home, a decent amount of research has to be completed. First take a horizontal measurement of the frame. To do this you should measure the distance across your frame at the top, at the middle and at the bottom. Write down the smallest measurement of the three figures. The smallest measurement is the one to reference when purchasing a window. Do the same thing when measuring the height. Take a measurement at the center of the window and at either side to see which section is the smallest. Keep the two smallest numbers, one for the width and one for height and rely on those to make your purchasing decisions.

You also have to measure the depth of the window opening. The depth is important because some windows will be too deep to fit into an opening. Run a tape measure from the outside of the frame to the inside and take the measurement from edge to edge. Do this in several different locations around the window and rely on the smallest figure.

Now that you have the measurements, you can use them to purchase windows that will fit into the space available. The windows should be approximately two inches narrower and two inches shorter than the measured space. This leaves an inch of space around each side of the window for positioning and adjusting the window to fit exactly where you want it. Use the depth measurement to ensure that your windows won’t be sticking out of the frame opening, the windows don’t have to be much smaller than the depth.

Measuring a Window Before Purchasing

While you’ll likely be given the measurements of any windows that you plan on purchasing, when you are able to see the windows in person you should measure them yourself to ensure that they are the right size for your opening before bringing them home. Bring your rough opening measurements with you and use them when purchasing your windows.

When you’re looking at used windows or floor models make sure that you take a measurement in person. Use a tape measure to measure the height of the window from top to bottom in three different places. Make note of the largest of the three measurements just in case the window isn’t uniform in height. Then measure across the window from outside jamb to outside jamb to see how wide the window is. Take three measurements and use the largest. Finally measure the depth of the window unit in several locations to see if it’s going to fit in your opening. Compare the window measurements to the space available in your rough opening to see if the windows are compatible.

Most of the time, you won’t have to do this when ordering windows because the measurements will be readily available. It’s still good practice to take window measurements whenever you have windows physically available to you, even if it’s just to double check someone else’s work. There’s nothing worse than getting windows home that don’t fit.

A contractor measures a window

Easy Installation

Windows that are sized properly are much easier to install than windows that aren’t the right size. It’s a simple matter to put wedges in around the window and get it level and plumb before nailing, screwing or stapling it into place. When the space around the windows isn’t right, the installation will be more complicated. Either there won’t be enough space to level and plumb the window and material will have to be cut away or the window will sit crooked. If there’s too much space, it will be hard to shim the windows and lock them into place properly.


Drafts are common when windows aren’t installed properly. Once you have successfully learned how to measure windows, this could be something to avoid. They occur for a few different reasons. The first reason is that there are gaps left open around the outside of the windows. This usually happens when there is too much space around a window and the gaps aren’t filled properly. It’s much more difficult to fill in the space when it’s larger than an inch.

Drafts are also caused when windows warp and the seals between panes breaks. This occurs when windows are crammed into too tight of a space and they expand and contract against the surrounding material.

Stable Windows

Only windows installed into an accurately measured hole are going to function properly over time. After you learn how to measure windows, the windows need some space between the glass and the frame to prevent issues due to expansion and contraction, but they have to be close enough to be anchored to the frame effectively. Usually about an inch of space around the windows is the perfect compromise. It’s enough space to keep the windows from pressing against the frame and causing problems, but it’s close enough to facilitate secure anchoring.

Consider Exterior Space

If you opt for a swinging window, such as a casement window or an awning window, consider how much exterior space is available, since how to measure windows will be different for different window types. These windows swing out from your home, and if you try to install one of them at the peak of your roof where overhangs get in the way, it work out well. If there is a possible obstruction around the outside of your window opening, make sure you account for it. Whether your home is built right next to the garage, or you have other parts of the home or yard to work around, you have to keep these things in mind. Take measurements for how much space the new windows will have to swing. Bring these measurements with you and double check that the windows will fit into that space when open.

Cutting a Rough Opening for a New Window

Sometimes you’re more interested in making an opening around a window that you already purchased. To do this you simply take the measurements of the window that you have using the steps laid out above and decide what depth, width and height you need for the window. The depth can be the same as the window measurements, but the height and width of the opening both need to be two inches larger than the window to account for one inch of space on all sides. This leaves you with space to make adjustments to your windows. The adjustments help keep the window level and plumb during installation.

With the opening cut to the proper specifications you’ll have to build a frame for the window to sit in. You’ll have to put in a header as well as cripple studs above the rough opening for your window, and you’ll also have to put in one or more cripple studs underneath the rough opening. This helps transfer the weight of your home down through the studs rather than through the window. It’s vital that you get a header and the cripple studs in place before trying to install a window in the rough opening, without these, you could put pressure on the window and cause it to warp and possibly break.

A contractor leveling a window

Leave It Up to the Pros

While you can attempt to install replacement windows on your own, there are a lot of things that can potentially go wrong. If you aren’t sure about any of the steps, or you don’t have much construction experience, it’s a good idea to leave the work up to a professional. It can be difficult learning how to measure windows and install properly. Letting a pro handle the work will give you peace of mind and you’ll know that your windows are going to function properly for a long time after the initial installation. Sure you’ll spend more money, but you’ll save time in the process and you should also benefit from a window warranty when a licensed professional handles the installation.

Now that you know how to measure windows and their openings properly you can buy with confidence.